1. Field of the Invention
The present invention relates to a time information obtaining apparatus, which receives a standard time radio wave to detect time information, and a radio timepiece provided with the time information obtaining apparatus.
2. Description of the Related Art
At the current time, the standard time radio waves are transmitted from radio stations in Japan, Germany, UK, Switzerland. In Japan, for example, amplitude modulated standard time radio waves are transmitted at 40 kHz and 60 kHz from radio stations in Fukushima and Saga Prefectures, respectively. The standard time radio wave that includes a time code bit string indicating date data (year, month, day, time) is transmitted every period of 60 seconds.
Timepieces (radio controlled timepieces) are in practical use, which receive the standard time radio wave including a time code bit string to detect the time code bit string, and correct a time counted within the timepiece based on the detected time code. The radio controlled timepiece is provided with a receiving circuit for receiving the standard time radio wave through an antenna, a band pass filter (BPF) for allowing only the standard time radio wave signal to pass through, a demodulating circuit for performing, for example, an envelope demodulating process to demodulate the standard time radio wave signal, and a processing circuit for detecting a time code included in the signal demodulated by the demodulating circuit.
The time code includes plural codes each appearing every unit time (1 second). The time code used in Japan includes a code “P”, code “0” and a code “1”, wherein the code “P” is a code of a duty 20%, which keeps a high level for first 0.2 seconds and then a low level for the remaining 0.8 seconds in the unit time, the code “0” is a code of a duty 50%, which keeps a high level for first 0.5 seconds and then a low level for the remaining 0.5 seconds in the unit time, and the code “1” is a code of a duty 80%, which keeps a high level for first 0.8 seconds and then a low level for the remaining 0.2 seconds in the unit time.
The code “P” is used as a marker indicating a beginning of one frame in frames of the time code and also used as a position marker indicating data sections including minute, hour, date, year data. The code “0” and code “1” indicate “0” and “1” in the binary system, respectively. The rising edge of each code corresponds to a “second” synchronizing point.
A conventional processing circuit synchronizes a demodulated signal at a rising edge, and binarizes the signal at a predetermined sampling period to obtain a binary bit string. The processing circuit measures a pulse width (a time length of a high level and a time length of a low level) of each code included in the obtained binary bit string, and determines depending on the measured pulse width, to which of the codes “P”, “0” and “1” such measured code corresponds, and obtains the time information from a string of the determined codes.
The time code is carried by an amplitude modulated radio wave of 40 kHz and/or 60 kHz frequency. The amplitude modulated radio wave is easy to reduce and/or come under the influence of external noises while traveling in a space among buildings, whereby the time code would be damaged.
In a technique disclosed in Japanese Patent 2005-249632 A, a demodulated signal is binarized at a predetermined sampling period (50 ms), whereby TCO data is obtained. A list of data groups is produced, each consisting of a binary bit string including 20 sampled bits each appearing everyone second. The data groups are added every sampling point to obtain a stepwise waveform data, from which a “second” synchronizing point is detected.
In the technique disclosed in Japanese Patent 2005-249532 A, even though some noises are included in the demodulated signal, it is possible to detect the “second” synchronizing point. But when so many noises are included that a waveform of an original data pulse cannot be reproduced, it is hard to detect the “second” synchronizing point from the produced waveform data.
The present invention has been made to overcome the technical disadvantages involved in the conventional techniques, and has an object to provide a time information obtaining apparatus, which can detect the “second” synchronizing point with a high accuracy independently of noise effects and signal intensity of a received radio wave, and a radio timepiece provided with the time information obtaining apparatus